1. Field of the Invention
The present invention relates to the structure of a push button switch used as a manipulating switch for various kinds of electronic equipment, and more particularly, to the structure of a dome-shaped movable contact having a reverse function.
2. Description of the Related Art
There are structures as shown in FIG. 5 to FIG. 7 as the structures of conventional push button switches. FIG. 5 is a sectional view of a housing type push button switch, FIG. 6 is a partially detailed view of a state in which a movable contact is in contact with a fixed contact, and FIG. 7 is a partially detailed view showing the movable contact and the fixed contacts of a sheet type push button switch.
In FIGS. 5 and 6, a housing 21 is formed of a molded material of a synthetic resin or the like in a box-shape with an upper surface opened, and fixed contacts 22 and 23, which are formed of a conductive metal such as brass or the like, are disposed on the inner bottom thereof. The fixed contacts 22 and 23 are buried in the housing 21 by a method of insert molding or the like, and connecting terminal 24 and 25, which extend externally of the housing 21, are formed at the other end sides of the fixed contacts 22 and 23.
A movable contact 26 is formed of a spring member such as stainless steel, phosphor bronze or the like in a reversible dome-shape, and the lower peripheral edge of the movable contact 26 is placed in the housing 21 in contact with one of the fixed contacts, that is, the fixed contact 22 disposed on the inner bottom of the housing 21. Further, the apex portion of the movable contact 26 is disposed in confrontation with the other fixed contact 23 with a predetermined interval therebetween, and a cylindrical projection 26a, which is formed in a cylindrical shape and projects upward, is disposed at the apex portion of the movable contact 26. The cylindrical projection 26a is formed integrally with the movable contact 26 with its bottom 26b facing upward and its peripheral edge 26c facing downward.
A stem 27 is formed of a molded material of a synthetic resin or the like and includes a press section 27a for pressing the dome-shaped apex portion of the movable contact 26, a locking section 27b locked to a cover 28 composed of a metal sheet and covering the opening of the housing 21 and a manipulating section 27c to be manipulated with a finger of an operator from the outside.
The operation of the above push button switch will be described. When the manipulating section 27c of the stem 27 is pressed and the stem 27 is pushed down thereby, the press section 27a presses the cylindrical projection 26a disposed at the apex portion of the movable contact 26 to thereby reverse the movable contact 26, so that the cylindrical projection 26a disposed at the apex portion comes into contact with the fixed contact 23. As a result, the fixed contact 22 is connected to the fixed contact 23 through the movable contact 26.
In this case, since the cylindrical projection 26a, which is formed in the cylindrical shape and projects upward, is disposed at the apex portion of the movable contact 26 and the peripheral edge 26c of the cylindrical projection 26a is in contact with the fixed contact 23, they are in contact with each other at a position from which dust 34, which tends to gather at the center 23a of the fixed contact 23, is avoided. Accordingly, the contact is stably performed and the reliability of the contact is enhanced as compared with the structure in which the apex portion is in direct contact with the center 23a of the fixed contact 23.
FIG. 7 shows a case in which the movable contact 26 is used in a sheet type push button switch. In the figure, a circuit board 28 is composed of an insulating laminated sheet formed of a phenol resin or the like, and fixed contacts 29 and 30, which are composed of an etched copper foil or the like, are disposed on the circuit board 28. The movable contact 26 is disposed on the fixed contacts 29 and 30, and the movable contact 26 is covered with a sheet member 31 formed of PET (polyethylene terephthalate) or the like. A spacer 32 is formed of an insulating material such as PET (polyethylene terephthalate) or the like in a sheet-shape and clamped between the circuit board 28 and the sheet member 31, thereby forming an accommodating section 32a in which the movable contact 26 is accommodated. Note that, although not shown, a plurality of the fixed contacts 29 and 30 and the movable contacts 26 are disposed on the circuit board 28.
While the above sheet type push button switch may be directly manipulated with a finger of an operator from above the sheet member 31, another stem (not shown) or the like may be disposed on the sheet member 31 and the push button switch may be manipulated through the stem.
However, in the structure of the movable contact 26 of the above conventional push button switch, the cylindrical projection 26a which projects upward is disposed at the apex portion of the dome-shaped movable contact 26 and is integrally coupled with the movable contact 26 with the bottom 26b thereof facing upward and the peripheral edge 26c thereof facing downward. Accordingly, since the interior of the movable contact 26 surrounded by the cylindrical projection 26a acts as a non-movable region when it is reversed, there is a problem that feeling in manipulation is adversely affected when the push button switch is manipulated.
Further, in the sheet type push button switch, since a gap 33 is made between the peripheral portion of the cylindrical projection 26a and the sheet member 31 by the existence of the cylindrical projection 26a which projects upward of the movable contact 26, an intimate contact property is bad and a problem resides in the reliability and outside appearance of a product.
Therefore, an object of the present invention is to solve the above problems and to provide the structure of a dome-shaped movable contact having a reverse function used in a push button switch, wherein it is possible that the movable contact comes into contact with a fixed contact at position excluding the central portion thereof where dusts is liable to gather, obtains high contact reliability, has good feeling in manipulation and is excellent in an intimate contact property with a sheet member covering the movable contact.